Cimetidine dynamics were studied in six multiple‐trauma patients receiving 300 to 600 mg every 6 hr for prevention of stress ulceration. Patients received cimetidine intravenously for a mean duration of 7.8 days. There was a positive correlation between cimetidine serum concentration and gastric pH; the correlation between intragastric cimetidine concentration and gastric pH was stronger. The association between therapeutic efficacy (pH>4) and serum concentration over 1 mcg/ml was significant. The association with gastric cimetidine concentration above 2 mcg/ml was significant.Clinical Pharmacology and Therapeutics(1984)35,50–54; doi:10.1038/clpt.19